Beneath its thick veil of clouds, Venus may be staging some spectacular volcanic shows. To a number of scientists, a strong string of circumstantial evidence has transformed active vulcanism on Earth's sister planet from intriguing speculation to a strong likelihood.

Researchers involved agree that the study of Venusian volcanoes would likely shed valuable light on how volcanoes work on Earth.

Evidence that has tipped the scientific scales toward active volcanic activity on Earth's sunward neighbor comes from the work of Larry Esposito at the University of Colorado at Boulder. He leads a team of scientists who have an instrument aboard the Pioneer Venus spacecraft, which has been orbiting the planet since December 1978. Their device measures particles in the planet's stratosphere. He presented his findings Monday at a press conference at the National Aeronautics and Space Administration's Ames Research Center here.

''When we arrived at the planet, we were surprised to see large concentrations of sulfur dioxide,'' Dr. Esposito explains.

Sulfur dioxide, along with water and carbon dioxide, is a major constituent in the eruption plumes of many volcanoes on Earth. But the Colorado scientist admits that the team's first reaction was not to credit these concentrations - 50 times those measured previously - to a volcanic eruption. Rather, he thought that earlier, earth-based measurements might simply have been wrong.

It was not until 1982, following the eruption of Mexico's El Chichon volcano, that this possibility was suggested. Esposito and his colleagues were studying data from a similar instrument on a satellite orbiting Earth and were struck by the similarities between these sulfur dioxide measurements and those from Venus.

On Earth, major eruptions can inject large amounts of sulfur dioxide into the stratosphere, where the particles float for a long time. It takes several years for these particles to sift back down to the earth and for the stratosphere to return to normal. This is precisely the sort of thing that Esposito's instrument recorded on Venus. Over the five years since the satellite has been in orbit, the sulfur dioxide level has fallen steadily.

''So, it seems to us that the most likely explanation for this is a large ejection of sulfur dioxide into the atmosphere. And, certainly if it were Earth, the most natural way to interpret this would be a volcanic eruption which occurred a matter of months before we went into orbit,'' Esposito concludes.

There could be other explanations, he cautions. But this seems to be the simplist and it dovetails nicely with several other, independent lines of evidence.

Because of the planet's perpetual cloud cover, it is impossible to photograph its surface with ordinary cameras. However, radar can be used to map its surface terrain in considerable detail. The first efforts to do this, using ground-based radar, revealed surface structure that looked distinctly volcanic in nature. Since then, this has been confirmed by radar-mapping efforts of the Pioneer Venus spacecraft as well as by Soviet Venera missions. ''The (geologic) information makes a compelling case that Venus has a number of large and small isolated volcanoes,'' says Harold Masursky of the US Geological Survey.

The largest feature on the planet's surface has been named Maxwell Montes. It is an old, apparently extinct volcano, a massif some 5,000 feet higher than Mt. Everest. In addition, there are two areas that appear to have much younger volcanoes. These have been dubbed Beta Regio and Alta. Beta Regio is a feature much like Earth's Hawaii Midway chain (except that it is not covered by water). Alta consists of twin volcanic peaks, somewhat like Mt. St. Helens in the United States or Sicily's Mt. Aetna, atop a high ridge.

''These features are clearly young, geologically. That means less than a million years old. However, there is not enough geological evidence to determine whether any of these are currently active,'' Dr. Masursky explains, adding, ''For that, we needed some independent confirmation, which is what Dr. Esposito has provided.''

Finally, there is a third set of observations that argue for an active Venus. With another instrument on the Pioneer Venus spacecraft, Dr. Fred Scarf of TRW Inc. has recorded a number of radio signals that are strikingly similar to those generated by lightning on Earth.

Plotting the apparent source of these signals, he found that they cluster over Beta Regio and Alta. Several years ago, he suggested that the source of these might be lightning strikes generated in volcanic plumes, a phenomenon common on Earth. However, by itself, this was not considered strong enough evidence of volcanic activity on Venus because it is possibile that the signals might result from normal atmospheric lightning striking the high peaks in these areas in unusual concentration.

If Venus is active, as the evidence suggests, it appears likely that Venusian volcanoes are not only large compared with those on Earth but also more violent. Venus is about the same size and density as Earth and so is thought to have about as much heat coming from its core to get rid of. On Earth, this heat is dispelled in a number of different ways. Molten rock is continually rising to surface on the floor of the ocean. This is where new crust is formed. It also moves around rigid pieces of earth's crust, called plates, and is released in earthquakes and volcanoes.

But Venus appears to have a thick and stationary crust. This could mean that it vents all its interior heat in volcanic eruptions. If so, volcanic eruptions of a scale seen on Earth only once per century may be occurring on Venus once a decade, Esposito speculates. He has calculated that the size of an eruption needed to put the amount of sulfur dioxide in the atmosphere that his team measured would be about the size of the 1883 eruption of Krakatoa in Indonesia. That cataclysm threw five cubic miles of rock and ash into the air and was so loud it was heard 2,200 miles away in Australia.

It will probably not be until 1988, when the US sends its next spacecraft to Venus, that this theory of volcanic activity on Venus will be definitely proved or disproved. This satellite, the Venus Radar Mapper, will provide scientists with a much more detailed picture of the planet's surface than is currently available.